1. Field of the Invention
The embodiments discussed herein are related to a construction of an objective, specifically to a construction for fixing a lens group in a tube with high accuracy.
2. Description of the Related Art
In an optical system which requires a high accuracy lens arrangement such as in an objective of a microscope, an individual lens group is adhered to a lens frame referred to as an inner frame. Then each of the inner frames is fixed in a predetermined position in a tube, and an entire optical system is configured.
Adjustment of the objective during assembly is performed by fine-tuning the position of the inner frame, to which the lens group is adhered, in the tube. Therefore, securing the lens group to the inner frame with high accuracy forms the basis for fixing the lens group in the tube with high accuracy.
When fixing the lens group to the inner frame, the lens group is adhered while monitoring the state of eccentricity of lenses included in the lens group. If the lens group consists of a cemented lens, then one lens is first adhered to the inner frame. Subsequently, remaining lenses are cemented to the lens that is adhered to the inner frame.
Generally in this method, a lens which is closest to an image, or a lens which is closest to an object, is selected as the first lens to be adhered to the inner frame. The first reason for this is to make the inner frame work as a foundation to support the lens group in operation such as assembling and measurement. In other words, the inner frame serves as the lowest layer, and each of the lenses of the cemented lens are cemented to each other in an accumulating manner.
The second reason is that the state of eccentricity may be easily measured. In this method, if the first lens is not adhered with high accuracy, such an error would also impinge on the accuracy of the position of the second and later lenses. Thus, the initial first lens must be adhered with high accuracy when securing the cemented lens to the inner frame. Therefore, it is preferable to initially adhere to the inner frame a lens which is closest to an image or an object and of which the state of eccentricity may be easily measured.